Effects of Heavy-ion Irradiation on Micr at Moderate Temperatures*

V-4 wt.% Cr-4 wt.% Ti alloy is a promising candidate material for first-wall and structural applications in magnetic fusion reactors. In the past, fast neutron sources were used to evaluate postirradiation properties of fusion candidate materials. The recent shutdowns of the Fast Flux Test Facility (FFTF) and the Experimental Breeder Reactor (EBR-II) left U.S. researchers without local experimental facilities for such tests. Under such circumstances, ease of experimental control, availability, and relatively low cost make heavy-ion irradiation an attractive alternative, provided its limitations are appreciated. We selected 3-MeV V" and 4.5-MeV Ni" ions to investigate the effects of irradiation on the microstructure of V-4Cr-4Ti alloy in the temperature range of 200 420°C. The main interest is in the evaluation of this alloy's dimensional stability and susceptibility to irradiation embrittlement. In this paper, we report results of ion irradiation experiments and compare them with available data on fast-neutron irradiation. From transmission electron microscopy (TEM) analysis of ion-irradiated specimens, we found that the dominant feature of the postirradiation microstructure was a high density of dislocation loops and pointdefect clusters. Density and defect size depended on irradiation dose and temperature. Precipitates and voidshubbles were not observed, even in specimens that were simultaneously injected with He and exposed to heavy-ion irradiation. Increased transport of point defects to internal interfaces was observed, as manifested by defect denuded zones along grain boundaries, Defect denuded zones along grain boundaries could lead to segregation of impurities and solutes and formation of precipitates on grain boundaries. INTRODUCTION Vanadium-base alloys are promising candidate materials for application in fusion reactors. Their advantage over other candidate alloys stems from their low intrinsic neutron activation, compatibility with Li reactor coolant, and refractory nature."* Results from past neutron irradiation experiments show that V-base alloys can be tailored to sustain higher operating temperature, stress, and neutron fluence than stainless steels, without suffering embrittlement or significant dimensional changes (~welling).~ These experiments were performed at temperatures above 420°C. Recent consideration of V-base alloys for applications as first-wall and structural material in the International Thermonuclear Experimental Reactor ( ITER ) prompted evaluation of the irradiation performance of these alloys in the temperature range from 200 to 420"C, which is also relevant to transient situations in other fusion reactors. Because operational fusion reactors are not available, the standard procedure to evaluate irradiation performance of materials that are relevant to a fusion environment is to perforrn irradiation experiments in fast-neutron spectra (E,>O. 1 MeV). In the past, the widely used devices were the 14-MeV Rotating Target Neutron Source RTNS and fast-fission reactors: i.e., the Fast Flux Test Facility FFTF and the Experimental Breeder Reactor EBR-11. However, these facilities are no longer available. The recent shutdown of EBR-I1 (September 1994) left U.S. experimenters without access to a local fast reactor. At this time, evaluation of the irradiation behavior of materials is limited to irradiation in mixed-neutron-spectrum reactor^^'^ (with their inherited excessive transmutations due to thermal neutrons), or to conducting experiments in foreign facilities6 (which extend costs and experimental time). In such a predicament, the ease of experi*Work supported by the Office of Fusion Energy, U.S. Department of Energy, under Contract W-3 1-109-Eng-38. *t Deceased in 1996. mental control, availability, and relatively low cost make heavy-ion irradiation an attractive alternative or supplement, provided its limitations are appreciated. The limitations include inhomogeneous profile of damage, small volume of irradiated material, different recoil spectrum than that of neutrons, and changes in target composition that are a result of atom injection. The main advantages of ion irradiation arise from the ease of experimental control (temperature, fluence, and dose rate), wide availability of irradiation facilities, relatively low cost, and, in some facilities, possibility of in situ ~tudies.~ From the family of V-base alloys, the V-4 wt.% Cr-4 wt.% Ti (V4Cr4Ti) composition gives superior irradiation performance.8 We selected 3-MeV V' and 4.5-MeV Ni" ions to investigate the effects of irradiation on the microstructure of V4Cr4Ti alloy in the temperature range of 200420°C. Microstructure was observed by transmission electron microscopy (TEM). The main interest is to evaluate this alloy's dimensional stability and susceptibility to irradiation embrittlement. In this article, we report on the progress of ion irradiation experiments and present limited, preliminary, TEM data that is available from the EBR-I1 X530 fast-neutron irradiation experiment conducted at ~370°C. EXPERIMENTS Procedures used to prepare the V4Cr4Ti alloy (Heat 832665, ANL ID BL71) have been described in detail elsewhere.' Composition of the alloy is given in Table I. The plate used to prepare TEM specimens for irradiation experiments was warm rolled (at 400°C) to a thickness of 1 mm and then cold rolled at room temperature to 250 pm. Standard 3-mm-diameter discs were punched from the 250-pm-thick sheet. Discs used for ion irradiation were ground to a thickness of =125 mm thickness before polishing and annealing. All discs were mechanically polished to a 0.050-pm surface finish and annealed in a UHV ion pumped furnace (e Pa) for 1 h at 1050°C (for ion irradiations) or 1125°C (for neutron irradiations). The result was a fully recrystallized material with an average grain size of 20-40 pm. Table I. Composition (impurities in wppm) of V-4Cr-4Ti alloy (Heat 832665, BL71) ANLID Cr Ti Cu S i O N C S P Ca C1 B BL-71 3.8 wt% 3.9wt% 4 0 783 310 85 80 <10 <30 <10 <2 <5 Ion irradiations were performed at the Argonne Tandem Accelerator facility operated by the Materials Science Division. Beams of 4.5-MeV Ni" and 3-MeV V" ions were produced by a 2MV NEC ion accelerator. During some irradiation runs, He was simultaneously injected into specimens at the rate of 5 appddpa by means of 0.35-MeV He" ion beams from the 0.65-MV NEC ion implanter. Vacuum in the irradiation chamber was maintained at 1 Pa. Separate ion irradiation runs were performed with specimens kept within +2"C at 200, 350, and 420°C. TEM foils were prepared by removing an 800-nm-thick section from the irradiated surface and backthinning specimens to electron transparency. The 800-nm depth was selected on the basis of TRIM Code (version 95)" simulations of ion deposition ranges and damage profiles. TEM foils allowed observation of regions with highest He and V atom deposition rate, sufficient irradiation damage rates, and insignificant Ni deposition. Neutron irradiations were completed during the last run of the EBR-11 reactor in AugustKeptember 1994. The details of this irradiation experiment and the specimen matrix are given in an earlier report.'' Specimens were irradiated in Li-filled capsules for 35 full-power days at =37O-39O0C in core position (flux of 2.4-1Ol5 n.cm-2.s-1, E>0.11 MeV), and attained a damage dose of =4 dpa. TEM discs used for the present work were randomly selected from a batch annealed at 1125°C. Discs were electropolished from both sides to electron transparency to avoid near-surface regions. Electropolishing of all irradiated TEM discs was accomplished with a South Bay Technology Single Jet Electropolisher 550B and electrolyte consisting of, by volume, 70% H,SO,, 15% CH,OH, and 15% C,H,,O, Butyl Cellosolve that was maintained near -10°C. TEM observations were performed with two transmission electron microscopes. The microstructure of ion-irradiated specimens were evaluated with a Phillips CM30 analytical TEM. Neutron-irradiated specimens were examined using a JEOL 1OOCX TEM equipped with thick window X-ray energy dispersive spectrometer X-EDS, in which work with radioactive materials is allowed. The size and number density of radiation-produced defects were measured with a Zeiss particle size analyzerkounter from prints of electron-micrographs that were obtained from foils of known thickness. Foil thickness was measured by electron energy loss analysis, calibrated by stereomicroscopy and foil tilting experiments. RESULTS The typical microstructure of ion-irradiated V4Cr4Ti at all temperatures consisted of a high density of “black dot” defect clusters and dislocation loops, on the order of 2.1022 m-3. Short dislocation segments were occasionally observed at 420°C and 10 dpa. Two types of dislocation Burgers vectors, ao and ad2<111>, were observed, with the latter one predominating. The typical microstructural evolution with increasing damage dose (0.5 to 5 dpa) at constant temperature (200°C) is shown in Fig 1. The effects of temperature (200-420°C) on the size of the dislocation loops and their number density is shown in Fig. 2 for specimens irradiated to 0.5 dpa. Examples of microstructure after irradiation at 350°C to 5 dpa with/without He are shown in Fig. 3. The effects of simultaneous He injection during ion irradiation were insignificant. For comparison with irradiated specimens, Fig. 4 shows the typical microstructure of V4Cr4Ti alloy after aging in the irradiation chamber during a typical ion irradiation run, Le., 8 h at 420°C in Pa vacuum. The microstructure is identical to that of the as-annealed material. Most of the ion irradiations were performed with Ni” ions. To determine the effects of Ni, if 205°C; (a) 0.5, (b) 2, and (c) 5 dpa. Defects imaged in dynamical two-beam condition with g = (01-1) near [Oll] foil orientation. (aY20O0C, foil thicknesi=50nm, and (b) 420°C, fo:l thickness =60 nm. irrsdiated V4Cr4Ti7 5 dpa at 350°C; (a) without He, (b) 25 appm of He injected. Figure 5. Lack of effects of Ni on microstructure produced by ion irradiation at 200°C to 0.5 dpa; (a) Ni'+, and (b) V' Figure 4. V4Cr4Ti allov after aging: in " V irridiation chamber for 8'h at 420°C, in Pa UHV vacuum. Figure 6. Defect-denuded zones near grain boundary of V4Cr4Ti alloy irradiated at 420°C to 0.5 dpa. any, a limited number of irradiations were carried out with 3-MeV V' ions. Figure 5 shows microstructures produced by both types of ions at 200°C and 0.5 dpa. No difference in defect structure was observed. Increased transport of point defects to preexisting dislocations, grain boundaries and precipitate/matrix interfaces was documented in ion-irradiated specimens. Defect-denuded zones, 2 100 nm wide, were most pronounced along grain boundaries and precipitate/matrix interfaces at 420"C, but were also present in specimens irradiated at 350°C (width ~ 4 0 nm). An example is